Trouble finding diode in Fender "The Twin"

[rascalrick]

I am working on a Fender The Twin amp and cannot find D108 (30v zener) anywhere on the 4 pcb's in the amp. I am pretty sure it needs replacing as the voltage on the D supply is 44vdc instead of 30vdc (or slightly less with various input configurations). Does anyone here know where that diode is located?

[mikeydee77]

it is the little yellow guy next to the vactrol LDR and near IC101, or am I wrong?

[rascalrick]

I thought my old eyes were failing me, but low and behold that part isn't on my board (at least at that location)!

[mikeydee77]

I see what you mean. I only have one version of the schematic 026059 rev N. I guess there are multiple versions then... Trawl the web and contact nice fender folks

[rascalrick]

Thanks, I'll keep searching. It's hard to find "the twin" stuff..most everthing comes up as just as the original twin. Why on earth did Fender name it The Twin???

[Jazz P Bass]

Can you post what documentation that you have indicating "D108"?

[rascalrick]

This is the corner block of the schematic that I have. I am not sure if this is the correct one for my amp. I have only found a couple online, and this one has the correct connection between V7B and the tone stack.

[rascalrick]

Here is the portion of the amp with D108.

[g1]

I have only found a couple online, and this one has the correct connection between V7B and the tone stack.

Are you saying there are others without the diode that have a different connection between V7B and the tone stack? They are for "The Twin" ? What revision letter are those schematics?

[rascalrick]

Sorry for the confusion. I meant that the schematic that I have is the corrected one that doesn't show a connection between the V7B plate and the tone stack. Apparently there were some earlier versions of the schematic that showed a direct connection that were corrected later. All I know is that the previous poster showed the board with the zener diode next to LDR101, and my board does not have it there. I still cannot find D108 anywhere in this amp. The voltage on pin 3 of the LM339 was 30vdc before I replaced it, but now with the new LM339 in place the D power supply is 44vdc, and I am trying to locate the diode to see if it has failed.

[g1]

Ok, seems there was a typo on some earlier schematics with regard to the tonestack, noted here: https://music-electronics-forum.com/...002#post644002

I would think the diode is in a different location, especially if you found 30V there before. Even on the schematics that show it, it is not listed on the parts list so it may have changed locations.
Could just be a bad connection to it as zeners usually fail low resistance rather than open circuit.
Locate R306 and C303 in the supply and see if it is around there. Then follow the trace from R306 to IC101/R138/R139.

* +44V may have cooked the new chip as they are rated for 36V max.

[rascalrick]

Yes, I suspect that the 44v has probably cooked the 339. I have done some tracing, but will do further tomorrow. It is quite the mystery, as I have visually inspected all 4 pcb's on the amp and so far can't locate it! Crazy. I know it has to be there somewhere!

[g1]

The "D" supply also feeds R1 and R2 in upper left of schematic, so have a look around there as well. If nothing else, I don't think there is any problem having 2 zeners in parallel so you could just tack one across C303. If the old one came back online, it shouldn't matter.

[J M Fahey]

Please post the schematic version related to YOUR amp.

In full, not little bits here and there.

Also a PCB closeup of the relevant area.

Are there any unpopulated PCB holes?

Or maybe it smoked or cracked and an earlier Tech removed it , without placing a new one there.

Or he put a regular diode, say 1N400x, thinking it was "oh, just a diode", which of course will not "zene".

Have seen worse choices made

[rascalrick]

I know my plans in the shop for tomorrow! Thanks for all the good ideas! BTW...love the sign "Never think up reasons not to check something"